This invention relates to circuit boards, and more particularly to an improvement of the heat radiation characteristic of a semiconductor element fixedly bonded onto the circuit board.
In order to cool a semiconductor element such as an LSI fixedly bonded on a circuit board, the following two methods have been proposed in the art: In one of the methods, as disclosed by Japanese Patent Application (OPI) No. 35373/1992 (the term "OPI" as used herein means an "unexamined application") a circuit board is formed by using a high thermal-conduction substrate high in heat conductivity, higher than 10 W/m K, to improve the heat radiating characteristic of an image sensor. The other method is an LSI mounting method in which, as disclosed by Japanese Patent Application (OPI) No's 123462/1992 and 219083/1992, a high thermal-conduction substrate and an electronic cooling element such as a Peltier element are used in combination.
One example of the prior art will be described with reference to FIGS. 7 and 8. FIG. 7 is a plan view outlining the arrangement of a conventional circuit board, and FIG. 8 is a sectional view taken along line 50--50 in FIG. 7.
As shown in FIGS. 7 and 8, a plurality of wire bonding pads 2, and a die bonding pad 3 are arranged on the upper surface of an electrically insulating substrate 1. The wire bonding pads 2 are connected through bonding wires to a bonding pad (not shown) of a semiconductor element 5 (indicated by the two-dot chain line), and the die bonding pad 3 is positioned where the semiconductor element 5 is to be provided. Furthermore, a heat conducting/radiating layer 9 is formed on the lower surface of the substrate 1.
The wire bonding pads 2, the die bonding pad 3, and the heat conducting/radiating layer 9 are formed by using an electrically conductive material such as a copper material according to the conventional method. The semiconductor element 5 is fixedly bonded to the insulating substrate 1 with a thermally conductive adhesive agent 6 such as an adhesive agent "H70E-4" manufactured by the EpoTek company. The heat conducting/radiating layer 9 is spread over the lower surface of the insulating substrate 1, serving as a grounding layer. In general, the heat conducting/radiating layer 9 is directly exposed in the air without being covered with "Solder-resist", or it is fixedly connected to a casing which is, for instance, made of a metal plate.
In the circuit board thus constructed, the heat generated by the semiconductor element 5 is transmitted through the thermally conductive adhesive agent 6 to the die bonding pad 3, and then through the insulating substrate 1 to the heat conducting/radiating layer 9 formed on the lower surface of the insulating substrate 1, thus being radiated into the air or the metal casing.
However, the above-described heat radiating method suffers from a problem that, since the insulating substrate 1 is low in heat conducting speed and large in thermal capacity, the temperature of the insulating substrate 1 is increased, so that the heat from the semiconductor element 5 is not sufficiently radiated.
Furthermore, the above-described method is disadvantageous in the following points: In the method, the substrate is expensive. Furthermore, in the method, it is necessary to use additional devices for cooling or heat-radiating the semiconductor element, and therefore it is difficult to reduce the weight of the circuit board; that is, it is difficult to provide a circuit board light in weight. In order to control the additional devices, the number of signal lines is accordingly increased, and the power consumption is also increased. That is, the method is not economical against the desire of the user.